1. Technical Field
The present invention relates to the field of Video on Demand (VOD), and more particularly to a method and system for workload management in VOD services.
2. Related Art
A VOD service relies on a network Internet Protocol (IP) infrastructure and is generally based on three main components: head end equipment, middleware equipment, and end-user equipment.
The first component of head end equipment is the platform responsible for presenting the live content available to the user and may include managing the dynamic information coming from a broadcaster for an Electronic Programming Guide (EPG). The head end may also provide the administration services for the encoders/transcoders used to transmit the video streams to the users.
The second component of middleware equipment ensures the digital content delivery and provides the portal and services such as VOD that could be installed closed to the Digital Subscriber Line Access Multiplexer (DSLAM). The DSLAM is responsible for connecting the PSTN line to the IP Backbone of the operator. The user could use the middleware to provide certain video services.
The third component of end-user equipment includes the elements installed at the point at which the content is viewed by a user. This may include such elements as residential gateway (Modem), a Set Top Box (STB), an IP or Analogue phone and a PC. Video services may be provided in the STB or the PC or in some other way.
The combination of the three components is called the “triple play” model and could be relevant in different environments such as mobile phones, PDAs and car embedded devices.
A broadcast system such as that described above is based on a multicast mechanism (one to many connections) and uses the IP Group Multicast Protocol RFC1112 to manage the user requests. Users requesting the streaming of the same Video are brought together to become members of a unique IP Multicast Group (IPMG).
On the other hand a Video on Demand service is based on a unicast mechanism (one to one connection) and uses the Real Time Streaming Protocol (RTSP) (see RFC 2326). When a user wants to view a video, a unicast peer to peer session is created between user equipment and a Video server. The means used to manage the interaction with the video such as “Pause”, “Fast backward”, “Fast forward” and “Play” is handled by the RTSP. A Unicast peer to peer session leads to a huge number of sessions which may be very expensive in terms of bandwidth and server charges but give greater user flexibility in terms of viewing capabilities and functions.
With respect to multicast broadcasts there are also some advantages and disadvantages. In a multicast session the users must all start the video streaming at the same time and the VCR functions described above (i.e. “Pause”, “Fast backward”, “Fast forward” and “Play) are no longer available.
Real Time Streaming Protocol (RTSP) is an Internet Engineering Task Force (IETF) proposed standard for controlling streaming media (see RFC 2326). It describes a set of messages that enable the efficient delivery of streamed multimedia over Internet Protocol (IP) networks.
RTSP works with established protocols, such as the Real Time Transport Protocol (RTP—see RFC 1889) and Hypertext Transfer Protocol (HTTP), to provide an integrated approach to streaming media over the Internet.
In VOD service, a user makes a request to a server to receive a media stream using VCR-style asset controls such as play, fast-forward, rewind and pause. To implement these actions, the server uses RTSP message pairs which consist of a user request and a server response. One user action can give rise to more than one Time streaming Protocol (TSP) message pair.
FIG. 1 shows the main types of message that may be used in the RTSP protocol.
A user requests a movie with the DESCRIBE and SETUP requests. The server responds with a DESCRIBE response and provides parameters related to the media, such as the audio header and duration. The SETUP message transmits transport parameters and establishes a session with a unique session ID. Once the movie is opened, the user can play it in normal, fast-forward, or rewind mode by sending a PLAY request with a Scale parameter that indicates the mode and speed. A PAUSE request may also be sent to pause the movie. Finally, a TEARDOWN request allows the user to exit from viewing the movie.
As previously indicated IP multicast provides an efficient one-to-many delivery service. To achieve such a delivery using IP unicast traffic, each datagram needs to be sent many times. To achieve one-to-many delivery using IP broadcast traffic, a single datagram is sent, but all nodes process it, even those that are not interested in it. Broadcast delivery service is unsuitable for inter-networks, as routers are designed to prevent the spread of broadcast traffic. With IP multicast, a single datagram is sent and forwarded across routers only to the network segments containing nodes that are interested in receiving it.
Historically, IP multicast traffic has been little utilized. However, recent developments in audio and video teleconferencing, distance learning, and data transfer to a large number of hosts have made IP multicast traffic more important.
The following describes the main details of IP multicast operations.
All multicast traffic is sent to a class D address in the range 224.0.0.0 through 239.255.255.255 (224.0.0.0/4). All traffic in the range 224.0.0.0 through 224.0.0.255 (224.0.0.0/24) is for the local subnet and is not forwarded by routers. Multicast-enabled routers forward multicast traffic in the range 224.0.1.0 through 239.255.255.255 with an appropriate Time to Live (TTL). A specific multicast address is called a group address.
The set of hosts that wish to receive multicast traffic at a specific group address is called a multicast group or host group. Multicast group members can receive traffic at their unicast address and the group address. Multicast groups can be permanent or transient. A permanent group is assigned a well-known group address. An example of a permanent group is the all-hosts multicast group, awaiting traffic on the well-known multicast address of 224.0.0.1. The membership of a permanent group is transient, only the group address is permanent.
There are no limits on the size of a multicast group. A host can send multicast traffic to the group address without belonging to the multicast group. There are no limits to how many multicast groups a host can belong to. There are no limits on when members of a multicast group can join and leave a multicast group. There are no limits on the location of multicast group members. IP multicast must be supported by the hosts and the routers of an IP inter-network.
IGMP is used by IP hosts to report group memberships to any neighboring routers that are multicast enabled. IGMP is implemented in the IP module as shown if FIG. 2A. IGMP messages are generally encapsulated in IP datagrams.
An IGMP v2 message consists of 64 bits, and contains the type of the message, a maximum response time (used only for membership queries), a checksum, and the group address as is shown in FIG. 2B.
The message types used for communication between a host and a router are defined by the first 8 bits of IGMP v2 message headers, and are shown in FIG. 2C.
One of the main problems which exist in the area of VOD is the amount of variation in the demand for service. This is particularly the case where a system allocates each user a unicast timeslot when they demand it. For example, take a typical weekday for a VOD service: in the morning and afternoon there may be a sporadic and relatively low number of demands. However, in the evening there may be many users wanting to watch a video. Also the demand may escalate dramatically at the week-end, if the weather is poor, if a new film is released or just because it does.
There have been a number of workload management solutions proposed which are intended to minimise the impact on bandwidth of unicast VOD services.
One main solution is to use a content delivery network (CDN) architecture. This uses load balancing on a cluster of servers and replication of the video on many servers (proxy cache) distributed in different locations which are closer to the user. This solution is costly and requires a sophisticated architecture including many additional servers, switches and engines.
Patching has also been used as a way to allow a user to join an existing multicast for some or all of the video. This involves downloading a portion to user Set Top Box (STB) from a dedicated patching multicast stream. In the meantime a regular multicast is downloaded to the buffer in the STB and stored until the patching stream has reached the starting point of the regular stream stored in the buffer. Then the buffer is emptied to allow the user to view the video from the regular stream and also reloaded with the following parts of that stream for the remainder of the video. Relying on extensive use of the buffer in the STB is not an ideal solution as manufacturers are more and more producing STBs without so much buffer.
Many algorithms have been proposed for providing stream merging, with the purpose of merging streams started at different times, in order to reduce bandwidth. These are very complicated and do not always give the user a good quality viewing as speeds of streams have to change or be delayed to permit the required merging. Also the bandwidth saving are generally quite minor.
Another solution is known as piggybacking. This is a hardware solution which again requires streams to be speeded up and slowed down in order to splice two streams which started at similar times. Again, in practice this is very difficult to achieve and the user can generally see the changes and thus the quality of service is effectively diminished.
Thus, there is a need for a solution to the problem of workload management which overcomes at least some of the disadvantages of known solutions.